In Texas, the age of consent is 17. If the relationship began before she was 17, then it would be illegal. See below the statute....
Sec. 21.11. INDECENCY WITH A CHILD.
(a) A person commits an offense if, with a child younger than 17 years and not the person’s spouse, whether the child is
of the same or opposite sex, the person:
(1) engages in sexual contact with the child or causes the child to engage in sexual contact; or
(2) with intent to arouse or gratify the sexual desire of any person:
(A) exposes the person’s anus or any part of the person’s genitals, knowing the child is present; or
(B) causes the child to expose the child’s anus or any part of the child’s genitals.
(b) It is an affirmative defense to prosecution under this section that the actor:
(1) was not more than three years older than the victim and of the opposite sex;
(2) did not use duress, force, or a threat against the victim at the time of the offense; and
(3) at the time of the offense:
(A) was not required under Chapter 62, Code of Criminal Procedure, to register for life as a sex
(B) was not a person who under Chapter 62 had a reportable conviction or adjudication for an offense
under this section.
(c) In this section,"sexual contact" means the following acts, if committed with the
intent to arouse or gratify the sexual desire of any person:
(1) any touching by a person, including touching through clothing, of the anus, breast, or any part
of the genitals of a child; or
(2) any touching of any part of the body of a child, including touching through clothing, with the
anus, breast, or any part of the genitals of a person.
(d) An offense under Subsection (a)(1) is a felony of the second degree and an offense under
Subsection (a)(2) is a felony of the third degree.